1. Field of the Invention
The present invention relates to the field of signal frequency measurement. More particularly, the present invention provides a method and apparatus for measurement of the frequency of clock pulses which are utilized in a central processing unit (CPU) of a microcomputer or the like.
2. Description of Related Art
FIG. 1 generally indicates the prior art method and associated apparatus which have been used in conjunction with clock pulse frequency measurement systems in a microcomputer system. A monostable switch 2 is used to control a variable logic device 4. The output from variable logic device 4 is used to select one of a plurality of clock pulses with frequencies f1, f2, f3, . . . fn which are input into multiplexer 6. Alternatively, the variable logic from variable logic device 4 can be set with a software instruction 8 via an interfacing circuit 10. Output fx from multiplexer 6 is sent to both a CPU 12 and a frequency detecting device 14. Frequency detecting device 14 may be provided with a display 16.
In prior art systems the frequency fx and, therefore, the indicated frequency from frequency measurement device 14 often drifts over a period of time due to variations of temperature, material properties, noise interference, and the like. Additionally, in many systems only the two most significant digits of frequency are displayed and the rest of the digits are ignored by the device. As a consequence, the indicated frequency may contain significant, non-trivial error, especially when the third, undisplayed digit is equal to or greater than 5.
A timing diagram for prior art systems is provided in FIG. 2 and compared to the invention described herein. The frequency of a series of clock pulses, shown on coordinate C, is to be measured. In the prior art, a time base shown on coordinate A would be utilized. The frequency counter counts the number of clock pulses occurring in the time base and the result is displayed as the frequency of the clock pulses. For example, if the time base is 1 .pi.s and 10 pulses are counted in the time base, the frequency of the clock pulses is 10 MhZ. It is often observed, however, that the frequency of signal C drifts. Therefore, the indicated frequency tends to be unstable. This instability may result from temperature variations, noise, and the like.
Patents related to the field of frequency measurement include Carlton, U.S. Pat. No. 4,580,281; Cook et al., U.S. Pat. No. 4,616,173; Yool, U.S. Pat. No. 4,257,003; and Haymaker, U.S. Pat. No. 4,456,876.